Ultrahigh-frequency electron discharge apparatus



E. FEENBERG Dec. 25, 1951 ULTRAHIGH-FREQUENCY ELECTRON DISCHARGE APPARATUS Filed Aug. 26, 1947 Y INVENTOR.

EggeneIWeene/y ATT0RNEY Patented Dec. 25, 1951 ULTRAHIGH-FR-EQUENCY ELECTRON DISCHARGE APPARATUS Eugene. Feenberg, Kew Gardens, N. Y., assigner to The Sperry Corporation, a corporation of Delaware 'Application August 26, 1947, serial No. 770,637

lows that this result may be attained by utiliz- 1 ing a bunching voltage having a periodic savvtooth waveform. A voltage excursion of savv- 'tooth Waveform, having a period of 21r, may be dened by the equation where T represents the time phase at which an electron passes the center of the bunching space and V1 is a constant.

If is the corresponding time phase quantity at the catcher, normalized to vanish when T vanishes, then for a peak alternating voltage V1 across the buncher resonator grids (assumed small in relation to the beam voltage Vo), 0 is :Input radian frequency,

L=Length of drift space between buncher and catcher resonators, y vo=Average electron velocity in drift space, and N=Drift time measured in cycles of the voltage V1.

A periodical representation of the voltage excursion defined by Fig. 1 is indicated by the Fourier series v=v1isin 'r-1/2 smzT-tl/3 sin 3T .i (si Thus 0=Tr sin T-1/2 sin 2T+ (4) Consideration of only the first three terms of Equation 4 and generalization of its rSt tvvo sine terms leads to Claims. (Cl. 315-6) 01 is substantially constant during each cycle and all electrons passing the buncher during a cycle reach the catcher at approximately the same instant. Under these conditions the efciency of the klystron is maximum. According to the present invention, an approximation to the desirable sawtooth bunching voltage Waveform isprovided in a very simple manner, as will appear hereinbelovv.

Accordingly it is an object of the present invention to provide a more efficient klystron device.

It is a further object of the present invention` to provide apparatus for producing, in a velocity-modulation electron-discharge device, a bunching voltage approximating a non-sinusoidal waveform.

It is a still further object of the present invention for providing a simple klystron utilizing a bunching voltage of approximately savvtooth Waveform.

It is yet another object of the present invention to provide a novel multi-resonator electrondischarge device. A

Calculations with Equation 4 show that, for a klystron frequency multiplier, in which the natural resonant frequency of the catcher resonator is substantially four timesy that of the buncher resonator, the eiiiciency factor F4 attains its maximum value at X=1.68 and Y=0.45 and provides a definite improvement over the maximum eiciency factor of a klystron multiplier utilizing a sinusoidal bunching voltage. The efficiency factor Fm (where m is the order ofthe harmonic to which the catcher resonator is tuned) for large values of m, attains its maximum value in the neighborhood of X=4/3 and Y=1/6 and greatly exceeds the maximum eiliciency factor of the corresponding klystron multiplier utilizing a sinusoidal bunching-voltage waveform. According to the present invention, an approximation of the bunching voltage Waveform required to cause the constants in Equation 5 to attain these values is provided in a very simple manner. f

Accordingly it is yet a further object of the Apresent invention to provide a klystron frequency multiplier of greatly increased efficiency.

In accordance with one aspect of this invention, these objects are attained by providing an improved vcomposite buncher in place of the conventional simple buncher resonator. Such a composite buncher is formed of a rstor input cavity resonator tuned to the input frequency, in combination with `a second oating resonator tuned in the neighborhood of a harmonic which is preferably twice the input frequency, these two resonators being separated by a drift-spacedenning member. The catcher or output resonator is then located spaced from the floating resonator by a second drift-space. As herein used, the term' fiioating resonatcrfmayibe dened asa 'resonator which does "not have Vany source of energy external to the klystron and which is not coupled to a load utilizing the output of the resonator; howeverracircuit-element may be coupled to the oating resonator, ysolely for affecting some electrical characteristic of the oating resonator. It is .positioned-at'apoint intermediate the input and output "resonators" at which the electron beam is as yet underbunched, that is, in a conditioninivvhich .the electron beam has not yet attained .the full density-modulated condition necessary for en- .ergy-extraction therefrom. The oating reso- Mnator is excited into oscil'lation'lsolely-` bythe Lun- *derbunchedfbeam' and reacts on the bea-mito ve- 1 locity-'modulate it f at f the secon"d"'harmonic` frequency. VVThe combination of' buncher resonator, driftrspace fand* floating resonator rmay :be `properly 'termed `-a -compositefbuncher `'since 'it -acts substantiallyfl-ike `a singlebunching lresonator'sto produce the desired 'f non'-sinuscliidalfoune-hingn voltage waveform.

YVForaiklystron.-arnpliien'lthe catcherfresonator is made to have`fsubstan-tially thefsamematural resonant frequency as the first resonator. For `va `k-lystron multiplier, the ycatcher resonator is "malde Vtoflhave ,-a natural *resonant frequency substantially am times `:that fof `'the frst resonator, MwherelfmV is theforder of 'the desired' harmonic.

The invention also relates to thenovel" features ^or principles of 'fthe instrumentalities 'described hereinywhether or'f-not such `are used fiorithe rstated obj ects; or in the t'stated' fields tor: combinations.

Fig. 1 islaV isohematicdiagram` "chan-improved for-m of 'rklys'tron `amplifier fin :accordance :with

the teachings herein.

"Fig F2 vis fa1similar1 schematic diagram of an improved frform -lof klystron ffrequencyy multiplier Aaccording'v to 'the' pres ent t invention.

Referring lto -Fig. "f1, the "klystron @amplifier i show-nftherein includessaf cathode f2 and; a lreen- 'trantbuncher'resonator`3 having 'a pair of`electron-permeable lwalls orA fgrids 4 "and 5 aligned `with 'catho`deli3- fBeyondfbuncher-'resonator 'is located afreentrantiioating 'r resonator .TB spaced "from bun'c'herresonator'S.by-,a"driftitube Vof "flengthLr-'and'h'aving apairzfelectronepermeable *walls or grids'fand GffalsoalignedwithcathodeZ. "Floating resonatori'has iawnatural resonantfrequency substantially twice' that o'f-buncher resonator I"-3. .Beyond @fioating l resonator 5- is carreentrant Acatcherrresonatori Ill:l spaced fromiioat-V ural resonant frequency substantiallyethe sameas-A fio 4 terminal I5' provided in catcher resonator I0. A similar coaxial line terminal I4 is provided for oating resonator E, to which an impedance element may be connected for controlling or changing some characteristic of the oating resonator 6, such as its shunt impedance, Q, resonant fre- ".,quencyfor toadjust .the l,gain between the input-and floating resonators. Y

Beyond catcher resonator I 0 is positioned a collector plate or electrode I9 for collecting the ,electronsfafter: their traversal of the resonators. This'collector electrode may be formed as a Wall of the device provided this wall is electrically infsulatedfrom cathode'Z.v

4Respective tuning rods I6, I1 and I8 are provided for resonators 3, 6 and I0 for independently .adjustingorituningthese resonators. Any other tuning means or resonator structure permitting ltuning may be employed, if desired.

In operation, the electron beam from cathode 2 traverses' buncherresonatorft :where it isfvelocity- 'modulated `by -the .electromagnetic 'eld" therein f whichv `produces fan-alternating. electric fleldxbetween electron-permeable Walls tand .5. 'The velocity-modulated .beam Ais partially hunched in `the 'drift tube 'I zand'. then'traversesthe floating resonator. Inasmuch ascomponentsof ylootlrthe frequency of `excitation of `Vlouncher resonator 3 and harmonics thereof are present inthe yelectron beam;floating'resonator 6 `is :excited into oscillationfat .thezsecondharmonic to whichit is r tuned. Thev oscillating'eld' Within the floating vresonatorf will in turn react on the electronbeam to velocity-modulate fthe velectron tbeam vfat-this second'harmonic.

After the Velectron beamy traverses the floating -resonatorfly :thefbunching vof the beam is;com '.-pleted in the driftitube I I feloeyondAv the-floating resonator 6. The bunchedfelectron beam-.then ,traverses the .catcher resonator I- to give up energy to the oscillating electromagnetic eld within 'the'catcherresonator I0. The load to'be'excited Eby' the'iamplified-energy is 'coupled to vthe output coaxial line terminal I5 provided in catcher resonator I'Il.

The I.inputresonator-13,l the 'drift tube'v 'I- and the Y iioating' resonator forml the Acomposite buncher Iof thefpresent invention. Despite the fact that "both-resonators 3 and 6 are excited by sinusoidal oscillations, their net effect is to velocity modulate the electron'beambetween cathode 2 and collector I9 in the .same manner as the electron beam would be modulated if it were. acted upon by a buncher excited by. a periodic voltage of sawtooth Waveform. 'husagpractical approximation to the'desired sav/toothwaveform for the buncher voltage is obtained, and as a result, lwhen the circuit parameters ,are ..properly proportioned, the .hunched electro-nsmay be caused totraverse the catcher according to the.v relationship shown Equatonl. ,The-values for. end-.Yer Equa- .tion 5..areob.tained by. Suitable selection 0r adjustment of the gain between input resonator 3 and floating resonator', and of the resonant frequency of resonatorl. As 'statedabove, maximum :eifciencyfor the klystron amplierlis obtained in the neighborhood of .73:2 and -y-=1, and has Abeen found to, bemuch greaterfthan the maximum eiciency of the two-resonator klystron-amplifier shown .iniFigJ vof U.- S. .Letters Patent No. 2,242,275 to R. H. Varian, and -`also to be greater than the maximumeiciency-of ,a three-resonator cascade ampliiier'of'the type shown in Fig. 1 of U. S. Letters Patent No. 2,280,824 to W. W. Han- Asen et al., fin Awhich-a. catingresonator is provided having the same natural resonant frequency as the other two resonators. The considerations leading to the proper selection of parameters will become clearer from the following discussion, for which it is necessary to define certain quantities, as follows:

N1=Average transit time in cycles betweeninput and floating resonator.

Nz=Average transit time in cycles between floating and catcher resonators.

Thus N1 and N2 are proportional to. L1 and L2.

lexp. jm] Where m is the order of the harmonic to which the catcher is tuned and the overline indicates an average over a full cycle of values of T.

Gn=the ratio of the floating resonator normalized alternating voltage a2 to the normalized alternating bunchervoltage a1 for B=0.

Hence where a1=V1/Vo, a2=V2/Vu, V2 is alternating voltage across the floating resonator'grids, R25 is the shunt impedance of floating resonator and Ro is the beam impedance which is equal to beam voltage Vo divided by cathode current In.

The following table shows the relation between these parameters,

[f or the optimum condition of X:2, Y=1.]

N2/N1 G0 B F1 Degrees 25 8. 4 15.0 763 50 5. 3 27. 5 759 1.00 3. 5 34.0 750 2.00 2. 8 46.0 748 4.00 2. 6 55. 0 752 A ratio Naz/N1 of 3.70 has been found to yield highly desirable results. Since the length of drift tubes 'l and II are directly proportional to N1 and N2 respectively, it follows that tube Il should be from 3 to 4 times as long as tube 1 for optimum results.

It will be observed that all values of the phase angle B for the conditions of operation listed in the table above are positive indicating that the high frequency voltage at the floating resonator leads the high frequency component of beam current which passes through the resonator. In order to attain this condition in operation, the floating resonator 6 of the apparatus disclosed in Fig. 1 must be tuned to a frequency slightly greater than twice the resonant frequency of res` onator 3.

These values of F1 may be compared with the peak value .58 for the conventional two-resonator klystron amplifier, leading to an increase in theoretical power output of approximately 70% for the present device.

- 6 "In each case, the optimum tuning ofthe floating resonator is a function of Go. However, near N2/N1=0.5, F1 is practically independent of Go for Go greater than 4.

The circuit parameters indicated in Fig. 1 may be proportioned so that the on-resonance gain Go when B=0 is the exact value desired, or the circuit parameters may be proportioned so that the gain Go is larger than the value desired and then the shunt impedance R25 of the floating resonators may be reduced by coupling a resistor to the coupling lll of the floating resonator. Desirably, a non-linear resistor, such as a germanium or silicon crystal or a thermistor may be used, so that the gain can be controlled as a functionvof the amplituderofoscillation, tokeep the apparatus near optimum operatingl condition.

Fig. 2 is a schematic diagram of improved frev quency multiplier using the principles of the present invention. The frequency multiplier of Fig. 2 includes a cathode 22 and a reentrant buncher resonator 23 of resonant frequency f and having a pair of electron-permeable Walls or grids 2li and 25 aligned with cathode 22. Beyond buncher resonator 23 is floating resonator 26 spaced from buncher resonator 2.3 by drift tube 21 and having a pair of electron-permeable Walls or grids 28 and 29 also aligned with cathode 22. The natural resonant frequency of floating resonator 26 is made substantially twice that of the buncher resonator 23. Beyond floating resonator 26 is catcher resonator 30 spaced from floating resonator 26 by drift tube 3| and having a pair ofyopposing electron-permeable walls or grids 32 and 33 aligned with cathode 22. It has a natural rese-nant frequency m which may be the fourth or .larger order harmonic of the natural resonant frequency f of the buncher resonator 23.

Beyond catcher resonator 3U there is provided an electron collector plate 39. Respective input and output coaxial line terminals 34 and 35 are provided in buncher resonator 23 and output resonator 36. Tuning rods 33, 31 and 38 are respectively provided in buncher, floating and catcher resonators 23, 26 and 30 for individually tuning the resonators. As in the apparatus of Fig, l, there is provided in floating resonator 26 a coaxial line terminal 40 to which an impedance element may be connected for controlling or changing some characteristic of the floating resonator 26.

In operation, energy of frequency j is fedv into buncher resonator 23 by means of input coupling loop 34. The electron beam from cathode 22 is velocity-modulated by the alternating ileld within buncher resonator 23 and is only partially bunched in the drift tube 2l. The underbunched electron beam, in traversing floating resonator 26, excites it into oscillation at the second harmonic 2f, which oscillations in turn react on the electron beam to velocity-modulate it at the second harmonic. Optimum bunching of the electron beam is effected in-the drift tube 3| and the desired output harmonicis extracted from the electron beam by the catcher resonator 30. The electron beam after traversing catcher resonator 30 impinges'upon and is collected by the collector plate 39 beyond the catcher resonator 30. As in the apparatus of Fig. 1 that portion of the apparatus of Fig. 2 including the buncher resonator 23, drift tube 2l and floating resonator 26 may be termed the composite buncher, and produces-the advantages of the present invention. By suitable gain and resonator tuning adjustments, the composite buncher can be adjusted N/Ni Gb B F4' rees 0325? 6.538 27.A 5 G10 0. -50 3.148 5. 5 607 1.0 v 2.35 11.5 .600 2.00 l. 90 29.0 581i i 4.100l 2.05 44.5 .583 y Here again, where'Go is too nigh, it may be reduced as` discussed relative vto the amplifier of Figi 1i InV general, in AFig'. 2 'as in Fig. 1, the floating resonator should not be tuned exactiy to the sec'- ondharmonic, but should b e' detuned therefrom indicated by the value of B in the `above tables. 'Ifhe values of the phase angle B for the condition's'of operation listed vabove indicate that for N/Nl ratios of 0.25 and 0.50 the floatingresonator Ztmust-be tuned to a frequency slightly less than twice the resonant frequency of resonator 23,-: and for biz/N1 ratios of LOOand'la'rger the -fioatifng resonator must be tuned to afrequency-'slig'htlygreater than twice the resonant frequency of resonator 23.

The maximum obtainable efficiency of A'the frequencymultiplier of-Fig. 2 has been ioundto be much greater than the maximum obtainable efficiency ofthe two `resonator k-liystron multiplierutilizing asinusoidal bunching voltage. i

Although only the vfirst and second harmonic terms ofthe Fourier expansion of Equation 4 wereused in determining the 'number of resohaters-'in the composite bunchers of Figs.y land-2, the invention is not restricted only to those two terms; Moreitha'ntwo termsmayf be used,1re sulting in a corresponding increase in theI number! of resonators' of progressively increasing harmonic order forming. the composite buncher. Also; although in the apparatus of Figs. l and 2 the first and secondvterms of thelFourierlexpa-nsion'of Equation 4 were used for the'tworesonator Acomposite bunchenth'e two terms fused' may-'beV the 'rstiand any other harmonic, -and the frequencies of the first and second resonators ofthetwo resonators ofY thecompositebuncher would th'enlbe'thefundamental and thev other harmonic respectively. However'these modifications do not realize all the 'advantages ofthe invntion.

Thus; by the provisionv in velocity-modulated amplifiers and multipliers of novel bunching apparatus capable of producing a desired nonsinusoidal affective punching-voltage waveform, more: efficient velocity-modulation electron-discharge Ydevices are produced.

Since many changes could b'emade in the above-construction and many apparently Iwidely din'erent embodiments yof y this invention could be made without departing from the scope thereof, `it isV intended that all matter contained in the above description or shown'in the accompanyingdrawings shall be interpreted as illustrative and not ina limiting sense- What is claimed is:

l. Ann ultra --high frequency electron dischai" ''device comprising means for producing anelectror'ibearn including ay cathode; a'- cavity rescnatorflihaving 'a'pi'ede'termin'ed natu'rlal lres'onantfrequency vandfhaving a pair of--electronpermeable walls aligned with said cathode; first meansdening an electron drift space beyond said first cavity resonator; a second and floating cavity resonator beyondsaid first electron-driftspace-deflning means and having a natural resonant frequency substantially twice that of said first 'cavity' resonator, said second cavity resonator having a pair of electron-permeable walls aligned" with said cathode, the drift space between the electron-permeable Walls of said rst andf'second` resonators being substantially less than the space required to cause the electron beam to attain maximum density modulation; second means beyond said second cavity resonatorfdening afurther electron drift space; and means beyond said second electron-driftspace-dening means for extracting energy of frequency other than that of the second cavity resonator from said electron beam.

2.v Apparatus as in claim i wherein said energy-extracting means is a cavity resonator having a natural resonant frequency other than that of said second cavityresonator.

3. Apparatus as in claim 1 in which said energy-extracting means is a cavityrescnator having a natural resonant frequency substantially the same as vthat of the first cavity resonator.

4. Apparatus as in claim 1 in which said energy-extracting means is a cavity resonator Vhaving a natural resonant frequency harmonically related to that ofl said first cavity resonator and greater than the frequency of said secondresonator.

5. Ultra-high-frequency velocity-modulation apparatus comprising a cathode, a first resonatorhaving a predetermined natural resonant frequency and having an electron-permeable portion aligned with said cathode, first means defining an electron drift space beyond said rst cavity resonator,V a second resonator beyond said first drift space and having a predetermined natural resonant frequency substantially twice that of'said first resonator, saidI second resonator having an electron-permeable portion aligned with said cathode, the drift space between ,the electron-permeable portions of said rst and second resonators being less than the space required to cause the electron beam to attain substantially maximum density modulation, second means defining an electron drift space beyond said second resonator, and an energy-extracting circuit beyond said second drift space for extracting energy from the electrons of av frequency different from the natural resonantfrequency of said second resonator.

6. yApparatus as in claim 5 in which said enorgy-extracting circuit is a cavity resonator having a natural resonant frequency substantially the sainev as that of said first resonator.

7. Apparatus as in claim 5 in which said energy-extracting circuit is a cavity resonator havinga' natural resonant frequency harmon-ically. related to that of the first resonator and greater than thatv ofthe second resonator.

8. A composite buncher for electron discharge devices vof the velocity modulation type having meansA forv producing an Velectron stream,comprising a rst cavity resonator along the path of said stream for velocity modulating said electron stream, aY second floating resonator-airing said path beyond said memberY andtuned to substantially twice the resonant frequency of said' first resonator, andl a drifttube member surrounding said path betweenl said first-and second resonators, said drift tube member having a length substantially less than the length required to cause said velocity-modulated electron stream to attain maximum density modulation.

9. A composite buncher as in claim 8 in combination with further cavity resonator means along said stream path for extracting energy from said stream at a desired output frequency dierent from that of said second resonator.

10. An ultra-high-frequency electron discharge device comprising means for producing an electron stream along a predetermined path, resonant means along said path for producing velocity modulation of the electrons of said stream, second resonant means spaced from said first resonant means and located at a point where said stream is in under-hunched condition for further velocity modulating said stream, said second resonant means being tuned to a frequency substantially twice the resonant frequency of said first resonant means, and resonant energy-extracting means spaced along said stream path from said second resonant means by a distance substantially three times the spacing between said first and second resonant means.

11. Apparatus as in claim 10, wherein each of said resonant means is a cavity resonator positioned in energy-exchanging relation to said stream.

12. Apparatus as in claim 11, wherein said second resonant means is tuned to a frequency greater than twice the resonant frequency of said first resonant means, and differing therefrom by an amount less than the band-width of said second resonant means, whereby the oscillating eld of said second resonant means is phase-shifted relative to the corresponding current component of .the electron stream coupled thereto. Y

13. A composite buncher for electron-discharge devices of the velocity-modulation type having means for producing an electron stream, comprising first resonant means along the path of said stream for producing velocity modulation of said stream, a drift tube member surrounding path beyond said first said resonant means, and a second resonant means along vsaid path beyond said member at a point where said stream is in under-hunched condition, said lsecond resonant means being tuned to a frequency greater than twice the resonant frequency of said rst resonant means and differing from said twice frequency by an amount less than the bandwidth of said second resonant means, whereby said second resonant means is adapted to be excited by said under-bunched stream with a phase differing from the corresponding current component of said stream, whereby the velocity modulation of said stream is enhanced to produce more eicient bunching of the electrons of said stream beyond said second resonant means.

14. An electron discharge device comprising means for producing an electron stream along a predetermined path, a rst cavity resonator along said path and having a predetermined resonant frequency, a second floating cavity resonator positioned along said stream path beyond said first resonator and tuned to a natural resonant frequency substantially equal' to but slightly greater than twice that of said first resonator, and a third output cavity resonator positioned along said stream path beyond said second resonator and tuned to a frequency different from that of said second resonator for extracting energy of said diiferent frequency from Said electron stream.

15. An electron discharge ldevice comprising means for producing an electron stream along a predetermined path, three cavity resonators positionedsuccessively along said path, the intermediate resonatorV having a resonant frequency substantially twice that of the first resonator, a drift-space-dening member connecting said intermediate and rst resonators, and a further drift-space-dening member connecting said intermediate resonator and the third resonator, the drift space defined by said further member being substantially three times that defined by said first member.

EUGENE FEENBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,222,901 Hahn Nov. 26, 1940 2,305,883 Litton Dec. 22, 1942 2,414,843 Varian Jan. 28, 1947 2,424,959 Alford Aug. 5, 1947 2,425,748 Llewellyn Aug. 19, 1947 

